1. Field of the Invention
The present invention relates to a coupling and in particular a coupling for joining together a socket and a spigot inserted into the socket.
2. Description of the Related Art
Coupling devices for joining together a socket and a spigot, such as in a pipe fitting, are well known. Many such couplings incorporate a locking ring retaining mechanism. An example of a coupling device incorporating such a form of retaining mechanism is disclosed in U.K. Patent Application No. 2,066,914 A. This describes a releasable coupling device comprising a fitting having an opening for receiving the end of a tube and a retaining ring located in the opening. The retaining ring has a plurality of axially extending, inwardly biased fingers adapted to engage an inserted tube to restrain it against axial movement out of the fitting. The device is provided with an unlocking collar which may be used to force the plurality of fingers radially outward, thereby disengaging the fingers from the tube.
U.S. Pat. No. 4,696,497 describes a coupling which can be used to connect a variety of fittings such as fluid lines and electric cables. The coupling comprises a tubular socket dimensioned to receive the end of a tubular spigot. The spigot has recesses formed near its ends which serve as detent means. A locking ring is inserted into the socket and comprises a plurality of radially resilient fingers which extend axially into the aperture of the socket, angled so as to have oblique portions extending towards the mouth of the socket. The diameter of the spigot is greater than that of an imaginary circle defined by the ends of the resilient fingers. The spigot is pushed into the socket, deflecting the resilient fingers which snap into the recesses. Axial removal of the spigot from the socket is prevented by the location of the fingers within the detente recesses. To remove the spigot it must first be rotated within the socket to cam the fingers out of the recesses.
Similar devices are known for retaining a cap on the end of a spigot in the form of a rod which serves for example as an axle in a childs toy. Such cap devices incorporate a cup-shaped casing defining a socket, the rim of which is bent inwards to define an annular slot which opens radially inwards. A spring steel disc is retained in the slot, a star-shaped aperture being cut in the disc so that the edges of the aperture define pointed spring fingers extending radially inwards. The casing can be secured on the end of a rod the diameter of which is greater than the diameter of the circle touched by the tips of the spring fingers, simply by pushing the rod into the aperture. This causes the spring fingers to deflect and bite into the rod so as to prevent its removal.
Each of the above described structures has the disadvantage that any axial load on the coupling which tends to pull the spigot out of the coupling is born by the force exerted by deflection of the spring fingers. The greater the load applied to the coupling, the more the spring fingers will bend. By the very nature of springs, a bent spring will store energy. As the load applied to the coupling is increased there comes a point when the energy stored in the spring fingers is sufficient to cause them to deflect into a shape which disengages them from the spigot. The coupling then fails.